In a conventional direct current motor, the commutating brushes are oriented around the shaft so that optimum torque is achieved when power is applied to the motor. Generally speaking, this means that the magnetic field established by the armature winding has a direction that is transverse to the permanent magnet field (or field established by the stator field windings). Since the tendency is for the two fields to orient themselves in the same direction, a rotational torque is generated and the motor spins. Because input power is commutated to the next set of windings as the armature rotates, rotational torque is maintained.
In typical embodiments of the present invention, a second set of brushes is added to the commutator that are oriented perpendicular to the motor's original brushes. When power is applied to the second set of brushes, a magnetic field is generated in the armature that is parallel to the stator field, and no rotational torque is established. Depending on the polarity of input power, the two magnetic fields can be oriented with or against each other. In the first case, the magnetic system is extremely stable, and the rotor is strongly attracted to the stator field and held in its position. In the second case, the magnetic state is unstable, and the result is a repulsion force where the armature is pushed out from the permanent magnet field. Hence, with the armature initially displaced slightly in an axial direction from the center of the permanent magnet field, the application of direct current power to the second set of brushes can produce a predictable axial motion in the armature.
In other typical embodiments, a single set of brushes are employed to produce a magnetic field in the rotor that has components perpendicular and parallel to the stator field. In this device, applying electrical energy to the brushes produces rotary motion and axial translation simultaneously. Such a device is useful in applications where the two motions are well-defined and fixed, such as in starter motors for internal combustion engines.
Other typical embodiments of the invention include brushless motors, where the commutation function is performed electronically. In these devices, the rotor typically houses permanent magnets spaced about the shaft, and a set of sensors detects the position of the rotor. By knowing the position of the rotor and the magnetic field structure associated with the rotor, the electronic commutating circuitry can be controlled in such a way as to produce stator magnetic fields that have components perpendicular and parallel to the rotor field. The result is a device comprising a single element that can be rotated and translated.
Motors having an inherent magnetic attraction between the rotor and stator (due to the permanent magnets and ferromagnetic components) have the added feature of an inherently stable axial position. In such devices electrical energy may be applied with a polarity that causes repulsion of the armature away from a first position within the stator field to a second position partly outside the stator field. However, if no other latching mechanism is used, the armature will automatically return to its original position when the electrical energy is removed. By incorporating an auxiliary permanent magnet that holds the armature in its second position, it is possible to establish two stable axial positions for the armature. Therefore, the linear actuation can latch in two positions, and the armature can be rotated in either position without requiring that power be applied to translation brushes at the same time.
The principles of this invention can also be applied to a variety of direct current permanent magnet motors having ironless core rotors. In such devices, the rotor typically consists of epoxy-bonded windings in a well-defined shape and includes little or no ferromagnetic material. Hence, little or no inherent magnetic attraction exists between the armature and the stator field. This feature may also offer some advantages in certain applications.
Devices according to the present invention can be used as dual-motion actuators. They may incorporate clutches between two different rotary actuations. For example, when the armature is in one position, it may drive a high-torque worm gear assembly. When it is shifted axially, the clutch disengages the worm gear and engages, for example, a high speed rack and pinion assembly. Thus, a single motor can be used to perform two very different motions.
One type of application for this invention is the use of a single, multipurpose motor to perform both the window winding and the door locking functions in an automobile. The motor assembly can be cost-competitive with conventional mechanical mechanisms so that it can be incorporated into low and medium priced cars as standard equipment. The cost of the multipurpose door motor assembly can be less than the cost of the multiple, single-function actuators currently used on higher priced cars to perform the window actuation and door locking functions. Additional cost savings may be achieved by reducing the complexity of the electrical and mechanical parts. Integrating two actuators into one housing and reducing the routing requirements of the wiring harness facilitates installation and reduces assembly costs.